The document has about 700 pages with user defined styles like Opskrif1 (for Heading level 1 headings).
It was successfully created using the previous LibreOffice version 6.3.4.
It now only incorporates the very first occurrence of Styles Title and Opskrif 1.
Trying Update Table of Contents also failed.
Background color of document is Black with white words.
Have you checked that your styles are part of the TOC feature? Go to Outline & Numbering tab of the style definition and read the Outline level. If it is Body text, fix it.
If your styles are already correctly configured, attach to your question a 1-page sample containing styles paragraph for examination.
Any dditional information should be provided in an edit of your question or a comment.
I took a copy of my file with white background and immediately got a perfect table of contents. It is strange, but the one with the black background odt file was converted from an htm file from MySword notes. My problem disappeared after the subsequent cleaning up I did. Then I opened and reworked the perfect table of contents. Any changes appeared perfect.
I therefore retract the problem.